Over recent years, alkaline storage batteries—especially nickel-hydrogen storage batteries—have come to be used as power sources for equipment of which high output is demanded, such as hybrid electric vehicles (HEVs) and pure electric vehicles (PEVs). The hydrogen storage alloy used for the negative electrode active material of nickel-hydrogen storage batteries is generally LaNi5 or other AB5-type rare earth hydrogen storage alloy, partially replaced with an element such as aluminum (Al) or manganese (Mn). Because such AB5-type rare earth hydrogen storage alloy contains Al, Mn or the like having lower melting points, segregated phases such as Al-rich phases or Mn-rich phases are known to be readily generated at the crystal grain boundaries and surfaces thereof.
It is also known that various different crystalline structures are obtained through combinations of AB2 and AB5 type structures. An example is the hydrogen storage alloy put forward in JP-A-2002-164045, which has an A2B7 type structure whereby a rare earth-nickel type AB5 type structure is made to contain Mg, and an AB2 type structure and the AB5 type structure are stacked over each other with a period of two layers. In this hydrogen storage alloy put forward in JP-A-2002-164045, improvement of the hydrogen intercalation-deintercalation cycle life is enabled. However, it is evident that the hydrogen storage alloy with the structure put forward by JP-A-2002-164045 gives inadequate high-output performance (assist output) and has unsatisfactory performance for high-output applications.
Accordingly, as a means of enhancement to high output, Japanese Patent No. 3241047 put forward a hydrogen storage alloy in which nickel-rich regions were provided in an AB5 type structure alloy surface and the surface activity point was increased.
However, when the foregoing hydrogen storage alloy put forward in Japanese Patent No. 3241047 was used to construct a hydrogen storage alloy electrode, and in turn the hydrogen storage alloy electrode was used to construct an alkaline storage battery, it became evident that the resulting battery did not have adequate high-output performance. This is because when an alkaline storage battery is constructed using a hydrogen storage alloy in which an AB5 type structure such as LaNi5 has been partially replaced with an element such as Al or Mn, the rare earth elements and Al, Mn or the like that are present in the surface layer of hydrogen storage alloy particles dissolve into the electrolyte as a result of charge/discharge of the battery subsequent to assembly.
As a result, alteration occurs in the crystalline structure of the surface layer of hydrogen storage alloy particles, alteration also occurs in the content ratios of the nickel present in the surface layer and present in the bulk, and a gradient of such nickel content ratio arises. More precisely, as a result of elution of the dissolved components such as the rare earth elements and Al, Mn or the like, the ratio of the nickel present in the surface layer becomes relatively large, while the ratio of the nickel present in the bulk becomes relatively small. Thereby, the crystalline structure of the hydrogen storage alloy in surface layer is altered and diffusion of hydrogen is inhibited, so that adequate high-output performance cannot be yielded.